was changing, was altering, was cheating (on someone), was unfaithful
Она не изменяла тебе, я точно знаю.
She didn't cheat on you; I know for a fact.
— Не говори никому (2006)
Эта женщина всегда изменяла своему мужу.
This woman was always cheating on her husband.
Она изменяла ему в браке, и забеременела от другого.
She cheated on him during their marriage and got pregnant by someone else.
— Безумно богатые азиаты (2018)